1. Chapter 19 Star Formation (Birth) Chapter 20 Stellar Evolution (Life) Chapter 21 Stellar Explosions (Death)
Few issues in astronomy are more basic than knowing how stars form. Here, in this visible-infrared combined image captured with the new wide-field camera on the Hubble Space Telescope, we see a highly detailed view of the largest stellar nursery in our local galactic neighborhood. Called R136 and located about 170,000 light-years away, this region of about 100 light-years across harbors a rich collection of hundreds of young blue stars amid reddish gaseous nebulosity abundant in hydrogen from which they formed, probably as recently as a few million years ago. (STScI)

2. 19.1 Star-Forming Regions
Star formation is ongoing. Star-forming regions are seen in our galaxy as well as others.
Figure Extragalactic Star Formation The giant star-forming region at the right, called NGC 604, is roughly 500 pc across. It is found in the nearby galaxy M33, displayed at the left on the much larger scale of 40,000 pc across. (R. Gendler; NASA)

3. 19.1 Star-Forming Regions
Star formation happens when part of a dust cloud begins to contract under its own gravitational force; as it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core.

4. 19.2 The Formation of Stars Like the Sun
Stars go through a number of stages in the process of forming from an interstellar cloud

5. 19.2 The Formation of Stars Like the Sun
Stage 1:
Interstellar cloud starts to contract, probably triggered by shock or pressure wave from nearby star. As it contracts, the cloud fragments into smaller pieces forming many tens or hundreds of individual stars.
Figure Cloud Fragmentation As an interstellar cloud contracts, gravitational instabilities cause it to fragment into smaller pieces. The pieces themselves continue to fall inward and fragment, eventually forming many tens or hundreds of individual stars.

6. 19.2 The Formation of Stars Like the Sun
Stage 2:
Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation.
Stage 3:
The interior of the fragment has begun heating and is about 10,000 K.

7. 19.2 The Formation of Stars Like the Sun
Stage 4:
The core of the cloud is now a protostar and makes its first appearance on the H–R diagram.
Figure Protostar on the H–R Diagram The red arrow indicates the approximate evolutionary track followed by an interstellar cloud fragment before reaching the end of the Kelvin–Helmholtz contraction phase as a stage-4 protostar. The boldface numbers on this and subsequent H–R plots refer to the prestellar evolutionary stages listed in Table 19.1 and described in the text.

8. 19.2 The Formation of Stars Like the Sun
Planetary formation has begun, but the protostar is still not in equilibrium—all heating comes from the gravitational collapse.
Figure Interstellar Cloud Evolution Artist’s conception of the changes in an interstellar cloud during the early evolutionary stages outlined in Table (Not drawn to scale.) Shown are a stage-1 interstellar cloud; a stage-2 fragment; a smaller, hotter stage-4 fragment with jets; and a stage-5 protostar. The duration of each stage, in years, is also indicated.

9. 19.2 The Formation of Stars Like the Sun
At stage 6, the core reaches 10 million K, and nuclear fusion begins. The protostar has become a star.
The star continues to contract and increase in temperature until it is in equilibrium. This is stage 7: The star has reached the main sequence and will remain there as long as it has hydrogen to fuse. Most important: Stars do not move along the main sequence! Once they reach it, they are in equilibrium and do not move until their fuel begins to run out.

10. 19.2 The Formation of Stars Like the Sun
The last stages can be followed on the H–R diagram:
The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact.
Figure Newborn Star on the H–R Diagram The changes in a protostar’s observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6, often called the Hayashi track. At stage 7, the newborn star has arrived on the main sequence.

11. 19.3 Stars of Other Masses
A protostar must have 0.08 the mass of the Sun (which is 80 times the mass of Jupiter) in order to become dense and hot enough that fusion can begin.
If the mass of the “failed star” is about 12 Jupiter masses or more, it is luminous when first formed, and is called a brown dwarf.

12. 19.4 Observations of Cloud Fragments and Protostars
The Orion Nebula has many contracting cloud fragments, protostars, and newborn stars
Figure Orion Nebula, Up Close (a) The constellation Orion, with the region around its famous emission nebula marked by a rectangle. The Orion Nebula is the middle “star” of Orion’s sword (see Figure 1.8). (b) Enlargement of the framed region in part (a), suggesting how the nebula is partly surrounded by a vast molecular cloud. Various parts of this cloud are probably fragmenting and contracting, with even smaller sites forming protostars. The three frames at the right show some of the evidence for those protostars: (c) false-color radio image of some intensely emitting molecular sites, (d) nearly real-color visible image of embedded nebular “knots” thought to harbor protostars, and (e) high-resolution image of one of many young stars surrounded by disks of gas and dust where planets might ultimately form. (Astrostock-Sanford; SST; CfA; NASA)

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14. 19.4 Observations of Cloud Fragments and Protostars
Protostars are believed to have very strong winds, which clear out an area around the star roughly the size of the solar system
Figure Protostellar Wind (a) The nebular disk around a protostar can be the site of intense heating and strong outflows, forming a bipolar jet perpendicular to the disk. (b) As the disk is blown away by the wind, the jets fan out, eventually (c) merging into a spherical wind. In contrast to this art, part (d) is an actual infrared image of a hot young star (at right) whose powerful winds are ripping away the disk (at left) surrounding a Sun-like star (at center). This system is located about 750 pc away in the star-forming cloud IC (SST)

15. Discovery 19-1: Observations of Brown Dwarfs
Brown dwarfs are difficult to observe directly, as they are very dim.. The difference in luminosity between the star and the brown dwarf is apparent.

16. 19.5 Shock Waves and Star Formation
Shock waves from nearby star formation can be the trigger needed to start the collapse process in an interstellar cloud
Figure Generations of Star Formation (a) Star birth and (b) shock waves lead to (c) more star births and more shock waves in a continuous cycle of star formation in many areas of our Galaxy. As in a chain reaction, old stars trigger the formation of new stars ever deeper into an interstellar cloud.

17. 19.5 Shock Waves and Star Formation
Other triggers:
Death of a nearby Sun-like star
Supernova
Density waves in galactic spiral arms
Galaxy collisions

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19. 20.1 Leaving the Main Sequence
Eventually, as hydrogen in the core is consumed, the star begins to leave the Main Sequence
Its evolution from then on depends very much on the mass of the star:
Low-mass stars go quietly
High-mass stars go out with a bang!

20. 20.2 Evolution of a Sun-Like Star
As the fuel in the core is used up, the core contracts and the core begins to collapse.
Hydrogen begins to fuse outside the core:
Figure Hydrogen- Shell Burning As a star’s core converts more and more of its hydrogen into helium, the hydrogen in the shell surrounding the nonburning helium “ash” burns ever more violently. By the time shown here (a little after stage 8 in Table 20.1), the core has shrunk to a few tens of thousands of kilometers in diameter, whereas the star’s photosphere is ten times the star’s original size.

21. 20.2 Evolution of a Sun-Like Star
Stage 9: The Red-Giant Branch
As the core continues to shrink, the outer layers of the star expand and cool.
It is now a red giant, extending out as far as the orbit of Mercury.
Despite its cooler temperature, its luminosity increases enormously due to its large size.

22. 20.2 Evolution of a Sun-Like Star
Stage 10: Helium fusion
Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse, through a three-alpha process:
4He + 4He → 8Be + energy
8Be + 4He → 12C + energy

23. 20.2 Evolution of a Sun-Like Star
Figure Reascending the Red-Giant Branch A carbon-core star reenters the giant region of the H–R diagram—this time on a track called the asymptotic-giant branch (stage 11)—for the same reason it evolved there the first time around: Lack of nuclear fusion at the center causes the core to contract and the overlying layers to expand.
The star has become a red giant

24. 20.3 The Death of a Low-Mass Star
The ejected envelope expands into interstellar space, forming a planetary nebula.
Figure Ejected Envelope A planetary nebula is an extended region of glowing gas surrounding an intensely hot central star (marked with an arrow here). The small, dense star is the core of a former red giant. The gas is what remains of the giant’s envelope, now ejected into space. (a) Abell 39, some 2100 pc away, is a classic planetary nebula shedding a spherical shell of gas about 1.5 pc across. (b) The brightened appearance around the edge of Abell 39 is caused by the thinness of the shell of glowing gas around the central core. Very little gas exists along the line of sight between the observer and the central star (path A), so that part of the shell is invisible. Near the edge of the shell, however, more gas exists along the line of sight (paths B and C), so the observer sees a glowing ring. (c) Ring Nebula, perhaps the most famous of all planetary nebulae at 1500 pc away and 0.5 pc across, is too small and dim to be seen with the naked eye. Astronomers once thought its appearance could be explained in much the same way as that of Abell 39. However, it now seems that the Ring really is ring shaped! Researchers are still unsure as to why a spherical star should eject a ring of material during its final days. (AURA; NASA)

25. 20.3 The Death of a Low-Mass Star
As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to glow.

26. 20.4 Evolution of Stars More Massive than the Sun
A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate.
Eventually the star dies in a violent explosion called a supernova.

27. 21.2 The End of a High-Mass Star
A high-mass star can continue to fuse elements in its core right up to iron (after which fusion reaction stops).
Figure Heavy-Element Fusion Cutaway diagram of the interior of a highly evolved star of mass greater than 8 solar masses (not to scale). The interior resembles the layers of an onion, with shells of progressively heavier elements burning at smaller and smaller radii and at higher and higher temperatures.

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29. 21.3 Supernovae
Carbon-detonation supernova: white dwarf that has accumulated too much mass from binary companion
Carbon fusion begins throughout the star almost simultaneously, resulting in a carbon explosion.

30. 21.3 Supernovae
Supernovae leave remnants—the expanding clouds of material from the explosion.
The Crab nebula is a remnant from a supernova explosion that occurred in the year 1054.
Figure Crab Supernova Remnant This remnant of an ancient Type II supernova is called the Crab Nebula (or M1 in the Messier catalog). It resides about 1800 pc from Earth and has an angular diameter about one-fifth that of the full Moon. Its debris is scattered over a region about 2 p. In A.D. 1054, Chinese astronomers observed this supernova explosion. The main image was taken with the Very Large Telescope in Chile, the inset by the Hubble telescope in orbit. (NASA)

31. 21.4 The Formation of the Elements
There are 81 stable and 10 radioactive elements that exist on our planet. Where did they come from?
Figure Elemental Abundance A summary of the cosmic abundances of the elements and their isotopes, expressed relative to the abundance of hydrogen. The horizontal axis shows each of the listed elements’ atomic number—the number of protons in the nucleus. Notice how many common terrestrial elements are found on “peaks” of the distribution, surrounded by elements that are tens or hundreds of times less abundant. Notice also the large peak around the element iron. The reasons for the peaks are discussed in the text.
This graph shows the relative abundances of different elements in the universe:

32. 21.5 The Cycle of Stellar Evolution
Star formation is cyclical: Stars form, evolve, and die.
In dying, they send heavy elements into the interstellar medium.
These elements then become parts of new stars.
Figure Stellar Recycling The cycle of star formation and evolution continuously replenishes the galaxy with new heavy elements and provides the driving force for the creation of new generations of stars. Clockwise from the top are an interstellar cloud (Barnard 68), a star-forming region in our Galaxy (RCW 38), a massive star ejecting a “bubble” and about to explode (NGC 7635), and a supernova remnant and its heavy-element debris (N49). (ESO; NASA)